docs: def _get_nonlinear_site_term(self, C, vs30, pga_rock):
          """
          Returns the nonlinear site scaling term (equation 7)
          """
          v_s = np.copy(vs30)
          v_s[vs30 > 760.] = 760.
          # Nonlinear controlling parameter (equation 8)
          f_2 = C["f4"] * (np.exp(C["f5"] * (v_s - 360.)) -
                           np.exp(C["f5"] * 400.))
          fnl = self.CONSTS["f1"] + f_2 * np.log((pga_rock + self.CONSTS["f3"]) /
                                                 self.CONSTS["f3"])
          return fnl